Remove an unnecessary OP_Null in the IN-operator logic. Attempt to clarify
comments explaining the IN-operator code, though it is not clear that the
comments are correct even yet - more work to be done.

Remove an unnecessary OP_Null in the IN-operator logic. Attempt to clarify
comments explaining the IN-operator code, though it is not clear that the
comments are correct even yet - more work to be done.
check-in: c11e55fa user: drh tags: trunk

Clean up the IN operator code generation logic to make it easier to reason
about. In the process, improve code generation to omit some unused OP_Null
operations.
check-in: 7c6fbcfe user: drh tags: trunk

** When IN_INDEX_MEMBERSHIP is used (and the b-tree will be used
** for fast set membership tests) then an epheremal table must
** be used unless <column> is an INTEGER PRIMARY KEY or an index can
** be found with <column> as its left-most column.
**
** When the b-tree is being used for membership tests, the calling function
** might need to know whether or not the RHS side of the IN operator
** contains a NULL. If prRhsHasNull is not a NULL pointer and
** if there is any chance that the (...) might contain a NULL value at
** runtime, then a register is allocated and the register number written
** to *prRhsHasNull. If there is no chance that the (...) contains a
** NULL value, then *prRhsHasNull is left unchanged.
**
** If a register is allocated and its location stored in *prRhsHasNull, then
** the value in that register will be:**** 0 if the (...) contains no NULL values** 1 if the (...) does not contain NULL values** NULL if we do not yet know if (...) contains NULLs**
** If the (...) does not remain constant for the duration of the query
** (i.e. the SELECT within the (...) is a correlated subquery) then the
** value of the allocated register is reset to NULL each time the subquery
** is rerun. This allows the caller to use vdbe code equivalent to the
** following:
**
** if( r[*prRhsHasNull] IS NULL ){
** r[*prRhsHasNull] = <test if data structure contains null>
** }
**
** in order to avoid running the <test if data structure contains null>
** test more often than is necessary.
*/
#ifndef SQLITE_OMIT_SUBQUERY
int sqlite3FindInIndex(Parse *pParse, Expr *pX, u32 inFlags, int *prRhsHasNull){
Select *p; /* SELECT to the right of IN operator */
int eType = 0; /* Type of RHS table. IN_INDEX_* */
int iTab = pParse->nTab++; /* Cursor of the RHS table */
int mustBeUnique; /* True if RHS must be unique */
Vdbe *v = sqlite3GetVdbe(pParse); /* Virtual machine being coded */
assert( pX->op==TK_IN );
................................................................................
int iAddr = sqlite3CodeOnce(pParse); VdbeCoverage(v);
sqlite3VdbeAddOp3(v, OP_OpenRead, iTab, pIdx->tnum, iDb);
sqlite3VdbeSetP4KeyInfo(pParse, pIdx);
VdbeComment((v, "%s", pIdx->zName));
assert( IN_INDEX_INDEX_DESC == IN_INDEX_INDEX_ASC+1 );
eType = IN_INDEX_INDEX_ASC + pIdx->aSortOrder[0];
if( prRhsHasNull && !pTab->aCol[iCol].notNull ){
*prRhsHasNull = ++pParse->nMem;
sqlite3VdbeAddOp2(v, OP_Null, 0, *prRhsHasNull);
}
sqlite3VdbeJumpHere(v, iAddr);
}
}
}
}
................................................................................
int rMayHaveNull = 0;
eType = IN_INDEX_EPH;
if( inFlags & IN_INDEX_LOOP ){
pParse->nQueryLoop = 0;
if( pX->pLeft->iColumn<0 && !ExprHasProperty(pX, EP_xIsSelect) ){
eType = IN_INDEX_ROWID;
}
}else if( prRhsHasNull ){
*prRhsHasNull = rMayHaveNull = ++pParse->nMem;
}
sqlite3CodeSubselect(pParse, pX, rMayHaveNull, eType==IN_INDEX_ROWID);
pParse->nQueryLoop = savedNQueryLoop;
}else{
pX->iTable = iTab;
}
return eType;

break;
}
/* Opcode: If P1 P2 P3 * *
**
** Jump to P2 if the value in register P1 is true. The value
** is considered true if it is numeric and non-zero. If the value
** in P1 is NULL then take the jump if P3 is non-zero.
*/
/* Opcode: IfNot P1 P2 P3 * *
**
** Jump to P2 if the value in register P1 is False. The value
** is considered false if it has a numeric value of zero. If the value
** in P1 is NULL then take the jump if P3 is zero.
*/
case OP_If: /* jump, in1 */
case OP_IfNot: { /* jump, in1 */
int c;
pIn1 = &aMem[pOp->p1];
if( pIn1->flags & MEM_Null ){
c = pOp->p3;

break;
}
/* Opcode: If P1 P2 P3 * *
**
** Jump to P2 if the value in register P1 is true. The value
** is considered true if it is numeric and non-zero. If the value
** in P1 is NULL then take the jump if and only if P3 is non-zero.
*/
/* Opcode: IfNot P1 P2 P3 * *
**
** Jump to P2 if the value in register P1 is False. The value
** is considered false if it has a numeric value of zero. If the value
** in P1 is NULL then take the jump if and only if P3 is non-zero.
*/
case OP_If: /* jump, in1 */
case OP_IfNot: { /* jump, in1 */
int c;
pIn1 = &aMem[pOp->p1];
if( pIn1->flags & MEM_Null ){
c = pOp->p3;

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